Team Fortress Underground

First off, assuming you're already slightly familiar with the game and how it works:
The most important thing in this game, without debate, is to be aware and actively responding to your teammates and the opposing team.

This is a rather vague statement is it not, well let me explain it further.

It's very important for you to be aware of what classes the rest of your team are playing as and how well each of them are doing. Usually, if you're playing on a 16 man server, there will be one class neglected already, so it's very important that the ones you and your team are playing as are helping you to victory instead of creating fatal weaknesses in your defensive and offensive strategies.
For example, if you have 3 pyros on your team, even though it's a downright destructive class to play as and you really like to burn people to death and want the achievements for it, DON'T PICK IT, go be a medic and heal them as they run into trouble or run as a distraction for your pyros by confusing the enemy with your jumping antics as a scout.
Sometimes, yes, it is advantageous to have multiple medics, maybe a couple soldiers, perhaps even multiple pyros and heavies. It's highly stupid, in my opinion, to have three spies or scouts, snipers or demomen, and escpecially Engineers, though, since they are highly specialized and don't lend themselves well to comprising an entire team.
Teams often work best, as a general rule of thumb, when they have 3 offensive/assault class players (the leftmost three classes on the selection screen), 3 defensive/guardian class players (the middle three classes), and 2 support role players (the rightmost three classes).
This balances things as best as I can see so that your team has the slower, heavier, more powerful characters moving in right behind the fast, agile, and most versatile classes while having enough, but not too many people working soley on the defense of an objective.
Now, obviously, this is not a rule to live and die by, you have to use your own judgement more than anything else. Just try to figure out what the opposing team is doing, what your objectives are, and what your team is doing, and pick your class and your duties accordingly. Also, work on becoming decent with all the classes, as it will come in handy. It also helps you understand the needs and playing style that will allow you to be a better team player when you're healing a heavy as opposed to a scout.

If you're interested in a more in-depth class guide, make sure to check this out.

Or give this a try if you want something more in depth that expands on what you've already read from this sight so far.

And here is yet another guide although I have to say it's the best of the three. That's part one and here's part two. Make sure to check both of these out if you're serious.

Then you can always go to GameFAQs with its huge database of game knowledge.